Sintered ferrite magnets are used in various applications such as motors, electric generators, speakers, etc. As typical sintered ferrite magnets, Sr ferrite (SrFe12O19) and Ba ferrite (BaFe12O19) having a hexagonal M-type magnetoplumbite structure are known. These sintered ferrite magnets are produced relatively inexpensively by a powder metallurgy method using iron oxide, and carbonate of strontium (Sr) or barium (Ba), etc. as raw materials.
To provide electric parts of automobiles, parts of electric equipments, etc. with reduced sizes and weights and higher efficiency for environmental protection, etc., sintered ferrite magnets are recently required to have high performance. Particularly demanded in motors used in electric parts of automobiles are sintered ferrite magnets having such high coercivity HcJ and high rectangularity Hk/HcJ that they are not demagnetized by a strong demagnetizing field even when made thinner, while keeping high residual magnetic flux densities Br.
To provide sintered ferrite magnets with improved magnetic properties, JP 10-149910 A and JP 11-154604 A propose methods for improving HcJ and Br by substituting part of Sr with rare earth elements such as La, etc. and part of Fe with Co in the above Sr ferrites.
Sr ferrites having part of Sr substituted by rare earth elements such as La, etc. for, and part of Fe substituted by Co, etc. (hereinafter referring to as “SrLaCo ferrite”), which are described in JP 10-149910 A and JP 11-154604 A, have excellent magnetic properties, so that they are widely used in various applications in place of conventional Sr ferrites and Ba ferrites. However, further improvement of magnetic properties is desired.
JP 2001-223104 A proposes a temperature-elevating speed of 1-5° C./minute in a temperature range from 900° C. to the highest temperature, in a temperature-elevating process in the sintering step, to improve the HcJ of the SrLaCo ferrite. JP 2001-223104 A describes in Examples that in the composition of Sr1-xLax(Fe12-yCoy)zO19, the HcJ is improved by a temperature-elevating speed of 1-5° C./minute. JP 2001-223104 A describes that the temperature-lowering speed is not particularly restricted, but usually 1-20° C./minute, specifically 5° C./minute in Examples. Temperature-elevating speeds in particular ranges in the sintering step for improving the HcJ of SrLaCo ferrites are proposed by JP 2000-277312 A, JP 2003-297623 A, etc. in addition to JP 2001-223104 A.
As sintered ferrite magnets, Ca ferrites are known in addition to the above Sr ferrites and Ba ferrites. It is known that Ca ferrites have a stable structure expressed by the composition formula of CaO—Fe2O3 or CaO-2Fe2O3, and that La added provides the ferrites with a hexagonal crystal structure. However, they have magnetic properties on the same level as those of conventional Ba ferrites, not sufficiently high.
Japanese Patent 3181559 discloses a Ca ferrite having part of Ca substituted by rare earth elements such as La, etc., and part of Fe substituted by Co, etc. for improved Br and HcJ, and improved temperature characteristics of HcJ, by having an anisotropic magnetic field HA of 20 kOe or more (hereinafter referring to “CaLaCo ferrite”). It describes that this anisotropic magnetic field HA is 10% or more higher than that of Sr ferrites.
However, CaLaCo ferrites have Br and HcJ on the same level as those of SrLaCo ferrites and extremely poor Hk/HcJ, despite a higher anisotropic magnetic field HA than in SrLaCo ferrites, failing to meet both requirements of high HcJ and high Hk/HcJ, so that they cannot be used in various applications such as motors, etc.
Various proposals have been made to improve the magnetic properties of CaLaCo ferrites. For example, JP 2006-104050 A proposes a CaLaCo ferrite containing La and Co at a particular ratio, in which the molar ratio of each element and n are optimized, WO 2007/060757 A proposes a CaLaCo ferrite, in which part of Ca is substituted by La and Ba, and WO 2007/077811 A proposes a CaLaCo ferrite, in which part of Ca is substituted by La and Sr.
However, the CaLaCo ferrites described in JP 2006-104050 A, WO 2007/060757 A and WO 2007/077811 A are insufficient for an increasingly stronger recent demand of higher performance, despite improved magnetic properties than those of the CaLaCo ferrite proposed by Japanese Patent 3181559. Thus, further improvement in magnetic properties is desired.
WO 2011/001831 A proposes the addition of more than 1% and 1.8% or less by mass of SiO2, and 1-2% by mass (as CaO) of CaCO3 as sintering aids to calcined bodies for the CaLaCo ferrites proposed by JP 2006-104050 A, WO 2007/060757 A and WO 2007/077811 A, thereby providing the CaLaCo ferrites with specifically improved HcJ while preventing decrease in Br and Hk/HcJ as much as possible.
Though the CaLaCo ferrites proposed by WO 2011/001831 A, which contains relatively large amounts of sintering aids, have specifically improved HcJ, most of them have as low Hk/HcJ as less than 85% at HcJ exceeding 360 kA/m (about 4.5 kOe), so that they cannot be made sufficiently thin.